It works perfectly fine from behind the camera.
I did some experiments using first black gaffers tape to cover the front sensor, then gaffers tape holding several folds of aluminum foil. It continues to work from behind the camera. Some people claim that there is an infrared sensor inside the viewfinder that gets a signal via the eyepiece, and this may well be true. I even covered this with gaffer tape and foil and it continued to work! Perhaps there were tiny gaps in my coverage that let signal through, but whatever the case it just works.
Removing all the foil and tape and taking it outside in bright sun, it continues to work. For the purpose of using it to trigger the camera when on a tripod, you can put the remote inches from the eyepiece hole and trip the shutter without camera vibration (which is the whole point of the exercise). So this little remote is a winner. We will have to see how long a battery lasts.
If you look at the viewfinder, you can see the sensor. It is not inside the viewfinder, but on the right side alongside it. This is what senses the presence of your face, but apparently it also receives the signal from the remote.
If the camera is set up to show you your photos (auto review mode) the remote shutter will be disabled for the 5 second review time, so shut that off if you expect to want to trigger the shutter in any kind of rapid sequence.
Tom's Digital Photography Info / tom@mmto.org